Pilot Relaying

Pilot Relaying

Abstract

In this paper, pilot relaying “a modified form of differential relaying” is presented. The goal of the paper is to explain the outlines of pilot relaying fundamentals and its applications.

Keywords: Pilot relaying, pilot wires, power line carrier, carrier current, microwave channel.

1 Introduction

Pilot relaying is an adaptation of the principles of differential relaying for the protection of transmission line sections. In a sense, it is a means of remote controlling the circuit breakers. The term “pilot” means that between ends of the transmissin line, there is interconnecting channel of some sort over which current and voltage information can be conveyed. Pilot relaying is used when high speed protection is required for all types of short circuits and for any fault location. This type of protection generally communicates the decision made by a local relay of one of the four types ( magnitude, directional, ratio, differential relays) to relays at the remote terminals of a transmission line.

2 Pilot Relaying

Pilot relaying systems employ high-speed protective relays at the line terminals in order to ascertain in as short a time as possible whether a fault is within the protected line or external to it. If the fault is internal to the protected line, all the terminal breakers are tripped practically simultaneously, thereby permitting high-speed reclosing. If the fault is external to the protected line, tripping is blocked.

The combination of high speed tripping and high speed reclosing has several advantages: (1) Decreased fault damage to equipment (2) Improved power system stability (3) Maximium return on the investment depending on improved stability (4) Decreased effect on nearby generation and load.

Pilot relaying is the best type of protection that can be applied to transmission lines. It is inherently selective and suitable for high speed operation.

Pilot relaying is used on some multiterminal lines where high-speed operation is not essential but where the configuration of the circuit makes it impossible for distance relaying to provide even the moderate speed. Also some lines are too short for any type of distance relay. For such lines the ohmic errors would be so high as compared with the ohms being measured that such relaying would be impractical.

Critical loads may require high-speed tripping beyond the capabilities of distance relays.

High-speed automatic reclosing of transmission line breakers after they have tripped to clear a fault is generally possible only with pilot relaying, because only pilot relaying is able to cause all the line terminals to trip at high speed and simultaneously. With such high-speed tripping and reclosing, generators do not have time to swing very far out of phase and therefore no synchronism check is necessary before reclosing. For these reasons it is suitable to use pilot relaying for most high-voltage transmission lines and for many subtransmission and distribution circuits.

“Pilot relaying is usually use to provide primary protection for a line. Back-up protection may be provided by a seperate sets of relays(step-distance relaying) or the relays employed in the pilot may also be used ” [1]

3 Pilot Channels

A communications channel is an integral part of a pilot relaying system that is used to protect a transmission line. “It is needed to deliver information from one terminal of the line to the other, so that a fault location can be determined.” [2] Similar information is also conveyed in the opposite direction, allowing simultaneous determination of the presence of an internal fault.

The types of pilot channels available for protective relaying are:

1)Pilot wires

2)Power line carriers

3)Microwave

3.1. Pilot Wires

Pilot wires are seperate wire circuits, generally consisting of a two-wire circuit of the telephone-line type, either open wire or cable; frequently such circuits are rented from the local telephone company. In some cases, the power systems may install their own wire pilot circuits.

“ The wire pilot channel, in all cases, is completely isolated from the power conductor and provides direct metallic connection between the terminals of the protected line – it is suited for both tripping and blocking pilots ” [3]

Having established that the purpose of a pilot is to convey certain information from one end of a line section to another in order to make selective tripping possible, the next consideration is the use to be made of the information. If the relaying equipment at one end of the line must receive a certain signal or current sample from the other end in order to prevent tripping at the one end, the pilot is said to be a “blocking” pilot. However, if one end cannot trip without receiving a certain signal or current sample from the other end, the pilot is said to be a “tripping” pilot.

Pilot wires can operate at power system frequency, audio frequency tones or in dc. Dc wire pilot relaying is not used than very special applications. “Ac wire pilot relaying is the most closely akin to current differential relaying.”[2] However, in modern ac-wire pilot relaying, the magnitude of the current that flows in the pilot circuit is limited and only a two-wire pilot is required. These two features make ac wire pilot relaying economically feasible over greater distances than current-differential relaying.

In ac-wire pilot relaying we should know two new terms to describe the principle of operation: “circulating current” and “opposed voltage”. Briefly “circulating current” means that current circulates normally through the terminal CTs and the pilot, and opposed voltage means that current does not normally circulate through the pilot. The schematic illustrations of the circulating-current and opposed-voltage principles are given in Figure 1 and Figure 2.

Figure 1. Schematic illustration of the circulating current principle a-c wire-pilot relaying

Figure 2. Schematic illustration of the opposed-voltage principle of a-c wire-pilot relaying

In both figures current-balance type relays are employed at each end. In Figure 2 the current transformers are connected in such a way that the voltages across the restraining coils at the two ends of the pilot are in opposition for current flowing through the line section as to a load or an external fault. Consequently, no current flows in the pilot except charging current, if we assume that there is no unbalance between the CTs outputs. The restraining coils prevent relay operation owing to such unbalance currents. But should a short circuit occur on the protected line section, current will circulate in the pilot and operate the relays at both end. Current will also flow through the restraining coils but in a proper application, this current will not be able to prevent relay operation; the impedance of the pilot circuit will be the governing factor in this respect.

Short circuits or open circuits in the pilot wires have opposite effects on the two types of relaying equipment,as the accompanying table shows.

Effect of short Effect of open

circuit circuit

Opposed voltage trip block

Circulating current block trip

Ac-wire pilot relaying is inherently immune to power swings or loss of synchronism than the dc.

In most cases, the application of wire pilots is limited to relatively short lines. “With increasing lengths, the shunt capacitance of the pilot pair, its resistance and cost will increase.” [3]. The distance limitation on the application of wire pilots is usually the attenuation of the signal due to the distributed capacitance and resistance rather than cost.

3.2. Power line carrier

Power line carriers use the protected transmission line itself to provide the channel medium for transmission of signals at frequencies of 30-300 kHz. These are the most commonly used “pilots” for protective relaying. Power line carrier relaying is also known as “carrier-current pilot relaying”.

In power line carrier relaying, the carrier transmitter-receivers are connected to the transmission line by line tuning equipment and coupling capacitor devices that are also used for line voltage measurement. Line traps tuned to the carrier frequency are located at the line terminals as shown in Figure 3. They prevent an external fault behind the relays from shorting out the channel by showing a high impedance to the carrier frequency and low impedance to the power frequency. The radio frequency (RF) choke acts as a low impedance to 50 Hz power frequency but as a high impedance to the carrier frequency. Therefore, it protects the equipment from high voltage at the power frequency and at the same time limits the attenuation of the carrier frequency.

“Since the carrier signal is propogated on the protected line itself, there can be no complete assurance that the signal will get from one terminal to the other when a fault is present on the protected line section in the path of the carrier signal. Hence it is better suited as a blocking pilot as the protected line is intact for an external fault when the carrier is transmitted.” [3]

Carrirer current pilot relaying is the best and most commonly used kind of relaying for high voltage lines. It is applicable in some form to any aerial line. It is preferred to wire-pilot relaying because it is somewhat more reliable and is more widely applicable. Consisting entirely of terminal equipment, it is completely under the control of user, as contrasted with rented wire pilot.

Figure 3. One-line diagram of power line carrier for pilot relay system. [1]

3.3. Microwave

Microwave channel is a radio channel which uses beamed radio signals of very short wavelengths (high frequencies in the range of 2-12 GHz), between line-of-sight antennas located at the terminals.

The basic microwave channel is subdivided or multiplexed so that it can be used for a large number of different functions simultaneously. When one of the subcarrier channels of a microwave scheme is used for protective relaying, it is usually modulated by frequency shift audio tone equipment. The protective relaying scheme operates in conjunction with the audio tone equipment which in turn utilizes the microwave subcarrier channel to propogate the tone signal to the remote terminals of the protected line. “A continious tone of one frequency, called the “ guard frequency ”, is transmitted under normal (or nonfault) conditions. When there is an internal fault, the audio tone transmitter is keyed by the protective relaying scheme so that its output is shifted from the guard frequency to a trip frequency.” [3]

Unlike power line carrier the microwave signal is propogated through the atmosphere. Such a system requires that a straight line from one antenna to another be above intervening objects; preferably above 50 feet. This usually limits the distance between antennae to about 20 to 50 miles depending on the topography of the land. “Transmission of a microwave signal beyond roughly 35 miles, requires a repeater terminal. This is simply a receive, amplify and retransmit terminal.” [2]

A typical microwave configuration is shown in Figure 4. The signals to and from the relays may or may not feed through the multiplexer. A higher level of dependability is achieved for the very important relaying function by working directly into the baseband equipment.

Figure 4. Typical microwave configuration [2]

Figure 5 shows the application that takes advantage of the reflective quality of the medium. A large billboard type structure is used to deflect the microwave signals to the desired path. The reflector terminal is entirely passive, providing no signal amplification whatever.

Figure 5. Use of a reflector [2]

Microwave has certain theoritical advantages over carrier –current because it is dissociated from the power line, but its only real advantage is in connection with remote tripping. Because the presence of a fault on the protected line will not interfere with the transmission of a remote-tripping signal. Occasionally, microwave is useful where the attenuation would be too high for carrier current, such as, on a power-cable circuit.

A microwave pilot is used only when the relaying equipment can share the channel with enough other services ; it is not economically justifiable for relaying alone if carrier current or wire pilot is applicable. Microwave is entirely suitable although it is not as reliable as carrier current for protective relaying purposes. This is because of the complex circuitry and the large number of tubes involved, and also because of large number of services on the same microwave channel.

4 Choice of Channels

Figure 6 provides a rough indication of the relative cost of several types of channels. Pilot wire channel cost increases linearly with distance, but it is the lowest-cost channel for short distances up to 5 or 10 miles beyond which a carrier-current pilot usually becomes more economical. A single microwave channel is not economically justifiable but as the diagram shows, the incremental cost for additional channels is small. Because the microwave is line-of-sight medium, and terrain and earth contours limit the distance over which transmission can take place reliably without repeat terminals, there is a jump in cost at the limiting distance.

“Power line carrier is not limited by distance, in general, but the terminal cost is appreciable because line traps, tuners, and coupling capacitors are required.” [2]

Figure 6. Relative costs of different channel types [2]

5 Pilot Relaying Schemes

Pilot relaying systems use either phase comparison or directional comparison to detect faults.

5.1 Directional Comparison

This type is usually applied over blocking type carrier pilot channel. In its basic application, directional phase and ground units sense the direction to a fault. They compare the direction of power flow at the line terminals. Power flow into the line at the terminals indicates that there is an internal fault, and the line is tripped. If power flows into the line at one end and out at the other, the fault is considered external to the protected line and tripping is blocked.

Consider the line shown in Figure 7. Assume that directional relays are used and high-speed protection is provided for the entire line by pilot relaying. Therefore, both faults F1 and F2 are detected as internal faults by the relays at B1 and B2 and cleared at high speed. Here, both relays see the fault current flowing in the forward direction. If this information is transmitted to the remote ends over a pilot channel, it is confirmed that the faults are the faults are indeed on the protected line. Now assume that there is a fault at F3 . The relays at B2 sees it as an external fault and the relays at B1 and B3 see it as an internal fault. Upon receiving this directional information at B1 , that relay will be able to block tripping for the fault at F3 .